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Foundations of Chemistry I - Honors
PERIODIC TRENDS IN REACTIVITY
DEPARTMENT OF CHEMISTRY
UNIVERSITY OF KANSAS
Periodic Trends in Reactivity
Introduction
The structure of the periodic table is such that elements with similar properties are aligned vertically in columns called “groups.” As you will learn in class, this leads to trends in properties
such as electron affinity and atomic radius as one moves both down the periodic table within a
specific group or as one moves horizontally along a given row in the periodic table. The reactivities of the elements and their compounds also follow well-defined trends both within the
group and across a given row. In this experiment, you will explore these trends in reactivity for
the elements for two different groups: the alkaline earths (group 2) and the halogens (group 17).
In addition, you will investigate the reactivity of a variety of different metals to generate an activity series for these elements. These trends can also be used to identify an unknown element.
The Alkaline Earths
The alkaline earth elements are all relatively reactive metals. Because of their electron configurations, these elements all tend to form dications (i.e. +2 cations) in their compounds. The solubility of the compounds formed from these cations with a variety of anions can be used to order
the alkaline earth elements according to their position within the periodic table. The relative solubility of the compounds with a variety of anions will be investigated in this experiment. The
trend in solubility varies smoothly among the alkaline earth elements. The solubility of compounds such as the sulfates, carbonates, oxalates, and iodates should either increase or decrease
gradually through this group. By observing these trends in solubility you should be able to confirm the ordering of the elements within this group.
The Halogens
The halogens, due to the their position in the periodic table, tend to form monoanions (i.e. -1 anion) in their compounds. They therefore gain electrons from other species, causing the oxidation
of that species. The oxidation of the halogen (A2) by the anionic halide (B-) is an example of
such an oxidation-reaction (redox) reaction. This reaction can be used to determine the relative
reactivity of the different halogens. Specifically, this reaction will reveal which of the halogens
is a better oxidant. Solutions of the different halogens and halides will be mixed together to determine which cases cause reactions and which do not. The reaction can be monitored using color changes.
𝐴! 𝑎𝑞 + 𝐵! (aq) → 2 𝐴! 𝑎𝑞 + 𝐵! (𝑎𝑞)
If A2 is a better oxidizing agent than B2, the above reaction will occur and color changes of the
solution will be observed.
Metal Activity Series
As described above for the alkaline earth elements, metals tend to lose electrons in combination
with nonmetals to form cations. These metals can react with oxygen, water, acids, or exchange
with other metals as illustrated in the example reactions in the scheme below. By exploring sev3
eral of these reactions with a series of metals, an activity series can be developed which describes the relative reactivity of different metals.
2Mg + O2 ! 2MgO (oxygen)
Ca + 2H 2O ! Ca(OH )2 (aq) + H 2 (g) (water)
Zn(s) + HCl(aq) ! ZnCl2 + H 2 (g) (acid)
Zn(s) + Cu 2 + (aq) ! Zn 2 + + Cu(s) (exchange)
Pre-lab
Pre-lab Assignment: Please answer the following questions in your lab notebook. This assignment is due at the beginning of lab. You will not be allowed to start the experiment until this assignment has been completed and submitted to your TA.
1)
Define the terms oxidation and reduction.
2) What physical or chemical properties would provide insight into which of the metals is
more reactive?
3) For the alkaline earth elements, determine the ionization energy for each of the elements.
Use these energies to comment on redox properties of the atoms.
Procedure
Safety: Goggles must be worn at all times. All solutions should be discarded in the waste container. Do not discard any solutions down the sink.
Part 1 – Alkaline Earth Elements
Solutions of nitrates of barium, calcium, magnesium, and strontium will be reacted with solutions of sulfate, carbonate, oxalate, and iodate. For each alkaline earth element, a series of four
different reactions will be explored. As you mix together the solutions described below, watch
for any changes that might indicate reaction including the formation of a precipitate and its appearance.
In each of the four test tubes, place ten drops of 1 M H2SO4. Add five drops of the barium nitrate solution to a test tube. Add five drops of the other solutions to their own test tubes such that
you have four test tubes of four different nitrates. Once the reactivity of the nitrate solutions of
the alkaline earths with sulfuric acid have been completed, rinse out the test tubes and repeat the
procedure above but add five drops of 1 M Na2CO3 solution to the test tube instead of the 1 M
H2SO4. Observe what happens when 5 drops of the nitrate solutions are added to these test
tubes. Repeat these four reactions for both solutions of 0.1 M (NH4)2C2O4 and 0.1 M KIO3. By
comparing the solubility of the different alkaline earth elements, determine if your results correspond with the ordering of the elements on the periodic table.
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Part 2 – Halogens
Caution: Avoid breathing the halogen vapors. Do not contact the halogen solutions with your
fingers, as chemical burns can result.
Place ten drops of bromine water to a test tube and add ten drops of hexane. Make sure you have
added enough hexane to see the layer separate from the aqueous layer and observe the color of
the hexane. Stopper the test tube and monitor the appearance of the solution. A color change
should occur in the hexane layer as the bromine is dissolved in the hexane. Note whether the
hexane layer is on the top or on the bottom. Carry out the same procedure for the chlorine and
iodine water. Note the colors of the different solutions and any changes that occur upon mixing.
Prepare three test tubes of each of the halogen solutions for a total of nine solutions. These solutions will be used in the next part of the lab.
You should now have three water/hexane solutions containing bromine. To one of the test tubes,
add a few drops of 0.1 M NaCl solution, to the next add 0.1 M NaBr, and to the third add 0.1 M
NaI. Stopper and shake each of these solutions and note any change in color that occurs. If a
color change occurs, this is evidence that a reaction has occurred in which the bromine has oxidized the halogen. Determine which halides bromine is able to oxidize. Repeat the above procedure using chlorine water and iodine water instead of bromine water. Record the relative oxidizing power of each of the halogens.
Part 3 – Metal Reactivity
Reactions with acids
Place 10 drops of 6 M HCl in each of six test tubes. Add a small piece of one of the metals to
each of the test tubes (Ca, Cu, Fe, Mg, Sn, and Zn). Monitor the solutions for evidence of reaction including gas evolution and color changes. Determine which of these metals react with HCl.
Exchange Reactions
In this part, you will determine whether each of the metals react with cations of the other metals
in solution. To each of seven test tubes, add a small piece of calcium. Then, add 5 drops of one
of 0.1 M solutions of the metal cations (Ca(NO3)2, Cu(NO3)2, Fe(SO4), Fe(NO3)3, Mg(NO3)2,
SnCl4, or Zn(NO3)2). Observe any changes that occur and note any indication of reaction, including color change and gas evolution. Repeat the above procedure starting with a small piece
of each of the metals for a total of 6 runs. Rank the metals in terms of increasing reactivity. Do
this by determining which metals replace others in solution. (Hint: start with the one for which
no replacement reaction occurs and end with the one that replaces all of the other metals in solution.)
Part 4 – Determination of unknown alkaline earth halide
Obtain a sample of alkaline earth halide unknown from your TA. From the experiments above,
develop a procedure for determining which halide and which alkaline earth cation are present in
your unknown sample. Carry out the identification of the unknown.
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Report
Your lab report should be a formal, individual report that addresses all the bullet points posed in
the rubric. It should be prepared according to the “Guidelines for Laboratory Reports” you have
been given.
References
This experiment is modified from the following sources:
“Experiment 12 The Alkaline Earths and the Halogens – Two Families in the Periodic Table” in
Chemical Principles in the Laboratory 9th Edition, E.J. Slowinski, W.C. Wolsey, Brooks/Cole,
Belmont CA (2009)
“Experiment 15 Activity Series” in Chemistry the Central Science Laboratory Experiments, J. H.
Nelson, K. C. Kemp, Prentice Hall, Upper Saddle River, NJ (2000)
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